1. Field of the Invention
The present invention relates in general to hydraulic pumps and motors and more particularly to such hydraulic pumps and motors having gerotor type gears.
2. Description of the Prior Art
Gerotor type pumps and motors are well known to those skilled in the art of hydraulic equipment. Gerotor pumps and motors utilize a set of gears the outer of which has teeth which face inwardly and the inner of which has teeth which face outwardly. The size, positioning, and arrangement of the teeth is such that hydraulic fluid cavities between the teeth open and close as the gears engage and rotate. This flow of hydraulic fluid can drive the rotation of a shaft connected to one of the gears (a motor) or can be driven by the shaft rotation (a pump). Gerotor pumps and motors are described in U.S. Pat. Nos. 4.501,536; 4,545,748; and 4,563,136.
In one type of gerotor pump or motor, fluid is conveyed to and from the gerotor gear set through a valve plate which has a circular array of openings therein. The valve plate and its circular array of openings rotates with the shaft and is disposed between the gerotor gear set and a commutator. The commutator has a circular array of inlet and outlet commutator openings. Rotation of the valve plate adjacent to the commutator causes the openings in the valve plate to pass adjacent to the inlet and outlet openings in the commutator creating fluidpaths from the openings in the commutator through the valve plate to the spaces between the teeth of the inner and outer gerotor gears. The inlet openings in the circular array of commutator openings alternate with the outlet openings. This alternating inlet and outlet arrangement together with the positioning of the commutator openings directs the proper flow of hydraulic fluid to and from the gerotor gear set. This type of gerotor hydraulic pump and motor with valve plate is described in U.S. Pat. Nos. 4,824,347; 4,699,577; and 4,813,856.
One of the difficult problems of gerotor type pumps and motors having valve plate and commutator directed fluid flow is that it is difficult to manufacture and assemble the commutator portion of the pump or motor. Moreover, regardless of the method of manufacture and assembly of the commutators in the past, the resulting fluid paths are relatively narrow creating a relatively large pressure drop in the hydraulic fluid as it moves through the pump or motor. Finally, one of the desired features for all pumps or motors of this type is a smaller size while maintaining a durable and strong construction. Improvements of these features have not been able to be achieved with the construction of the commutators as known in the prior art.
To achieve the construction of the prior art commutators having alternating inlet and outlet openings disposed in a circular array has required a difficult construction. First, an exterior housing piece is molded and machined with an opening for the shaft to extend axially therethrough. An inlet and outlet opening are provided on one side of the exterior housing piece and extend into the housing generally radially. A cylindrical cavity is provided in the exterior housing piece which extends coaxially with the shaft opening in the position desired for the commutator piece. The interior of this cylindrical cavity must be carefully machined to a precise size in order to receive a precisely sized commutator piece. The precisely sized commutator piece has the alternating inlet and outlet openings extending axially therethrough. This piece can be molded with this form and then machined to fit precisely within the cylindrical opening of the external piece of the housing. The commutator and housing pieces retain their precise assembled orientation by means of an irterference fit. This is accomplished by heating the external housing, precisely inserting the commutator piece and allowing the assembly to cool.
When subjected to extreme pressure or abusive conditions, the commutators of motors connected in accordance with the prior art can protrude from the housing since this connection is an interference fit. This protrusion can result in a loss of efficiency or even seizing of the motor.
To join the inlet and outlet paths of the external piece of the housing of the prior art with the axially extending commutator openings of the commutator piece, axially spaced annular openings are provided on the exterior of the commutator piece and the interior of the cylindrical opening which receives the commutator piece. Obliquely angled ports are drilled from the inlet and outlet ports of the exterior piece of the housing into the annular openings formed between the commutator piece and the external housing piece. These ports must be obliquely angled because the annular spaces are axially disposed with respect to each other preventing the annular openings from being directly beneath the inlet and outlet ports of the housing.
As described, the commutators of the prior art are difficult to manufacture because of the tortuous pathways therein. Moreover, this construction requires that the openings be relatively small. This combination produces a relatively high pressure drop as the fluid flows therethrough. This undesirable result is exaggerated if the size of the pump or motor is reduced.